1. Field of the Invention
The present invention relates to a discrimination method for discriminating optical discs having different thicknesses of transparent layers for which the same wavelength of laser light is used and to an optical disc apparatus using the discrimination method.
2. Description of Related Art
An optical disc apparatus is known well as using an optical disc as a recording medium for storing audio data and/or video data. The optical disc apparatus can read and write information on the optical disc by projecting a laser beam to the same.
The optical disc apparatus always detects a focus error signal for focus control. The optical disc apparatus performs a focus pull-in process based on the focus error signal so that a condensed light spot of the laser beam is focused just on a recording surface of the optical disc.
The focus pull-in process is performed as follows. The optical disc apparatus projects the laser beam onto the optical disc through an objective lens, and moves the objective lens to approach the optical disc while detecting the focus error signal. When an amplitude of the focus error signal crosses zero after exceeding a predetermined threshold value, it is determined to be a just focus point. Then, the movement of the objective lens toward the optical disc is stopped.
The optical disc apparatus is designed to be capable of reading and writing information on multiple types of optical discs, and it has a function of detecting which type of optical disc is loaded to the optical disc apparatus. In particular, a CD and a DVD are read and written by different wavelengths of laser beams, so it is very important to discriminate a type of the loaded optical disc.
The optical disc apparatus performs the above-mentioned focus pull-in process with respect to the loaded optical disc by using a laser beam for a CD and a laser beam for a DVD separately, and detects the focus error signal for each laser beam. Then, the optical disc apparatus discriminates a type of the loaded optical disc in accordance with whether or not the focus error signal satisfies the above-mentioned condition (i.e., to cross zero after exceeding a threshold value). For example, when the focus pull-in process is performed by using an optical system (a laser beam) for a CD, a point satisfying the above-mentioned condition (a just focus point) appears if the loaded optical disc is a CD while the above-mentioned condition is never satisfied if the loaded optical disc is a DVD (see JP-A-2007-12177, JP-A-2006-268958 and the like).
On the other hand, optical disc apparatuses supporting reading and writing information on a BD or an HD-DVD having a larger storage capacity have come on the market recently. Since the BD and the HD-DVD have higher recording densities, a laser beam having a shorter wavelength than the laser beam for a DVD is used for reading and writing information on the BD and the HD-DVD. Also for discrimination among a CD, a DVD, a BD and an HD-DVD, the above-mentioned discrimination method is used in which the focus pull-in process is performed with different wavelengths of lasers.
Although the same wavelength (405 nm) of laser light is used for a BD and for an HD-DVD, a thickness of a transparent layer is different between them (a BD has about 0.1 mm thickness, and an HD-DVD has about 0.6 mm thickness). In other words, a BD and an HD-DVD have different spherical aberrations due to their different thicknesses of transparent layers. Therefore, an optical pickup has an aberration correction unit for correcting the spherical aberrations of a BD and an HD-DVD so that the laser beam can be focused on each of the media.
However, if the focus pull-in process is performed by using the laser beam for which the spherical aberration correction is performed to adapt to one of the optical discs in case of optical discs such as a BD and an HD-DVD for which the same wavelength of laser light is used in the same manner as the case where the different wavelengths of laser beams are used as described above, the above-mentioned just focus point where a value of the focus error signal exceeds the threshold value and then crosses zero may appear in each of the optical discs. In this case, it is difficult to determine correctly a type of the loaded optical disc.